C/O Ratios And The Formation Of Wide Separation Exoplanets

By Keith Cowing
Status Report
June 19, 2024
Filed under , , , , , ,
C/O Ratios And The Formation Of Wide Separation Exoplanets
Origin of the line emission in the HD 163296 disk as determined in Guzmán et al. (2021) and Ilee et al. (2021). Also shown are the approximate C/O ratios derived from the analysis of Bosman et al. (2021a, from C2H) and also Calahan et al. (2023, from CH3CN and HC3N). — astro-ph.EP

The gas and solid-state C/O ratios provide context to potentially link the atmospheric composition of planets to that of the natal disk. We provide a synthesis of extant estimates of the gaseous C/O and C/H ratios in planet-forming disks obtained primarily through analysis of Atacama Large Millimeter Array observations.

These estimates are compared to atmospheric abundances of wide separation (> 10 au) gas giants. The resolved disk gas C/O ratios, from seven systems, generally exhibit C/O > 1 with sub-solar, or depleted, carbon content.

In contrast, wide separation gas giants have atmospheric C/O ratios that cluster near or slightly above the presumed stellar value with a range of elemental C/H. From the existing disk composition, we infer that the solid-state mm/cm-sized pebbles have a total C/O ratio (solid cores and ices) that is solar (stellar) in content.

We explore simple models that reconstruct the exoplanet atmospheric composition from the disk, while accounting for silicate cloud formation in the planet atmosphere. If wide separate planets formed via the core-accretion mechanism, they must acquire their metals from pebble or planetesimal accretion. Further, the dispersion in giant planet C/H content is best matched by a disk composition with modest and variable factors of carbon depletion.

An origin of the wide separation gas giants via gravitational instability cannot be ruled out as stellar C/O ratios should natively form in this scenario. However, the variation in planet metallicity with a stellar C/O ratio potentially presents challenges to these models.

Edwin A. Bergin, Richard A. Booth, Maria Jose Colmenares, John D. Ilee

Comments: 17 pages, 6 figures, accepted by Astrophysical Journal Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2406.12037 [astro-ph.EP] (or arXiv:2406.12037v1 [astro-ph.EP] for this version)
Submission history
From: Edwin A. Bergin
[v1] Mon, 17 Jun 2024 19:09:30 UTC (1,198 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻